Even at 0.5 micrometers design rules, the specifications on 5X reticles are set extremely tightly, on the grounds that ULSI patterns are so complex that tight specifications are essential to obtain acceptable yield. It is usually assumed that these specifications scale with the design rules, and that they should be even tighter for 1X reticles. As a consequence, it has been argued that 1X reticles for 0.25 micrometers design rules are impracticable. A statistical analysis, starting from first principles, and assuming point independent, normally distributed errors, supports the way in which mask specifications are currently set. The assumptions of spatial invariance and normal distribution are crucially important in the analysis. However, it is far from clear that they are valid. Consequently, mask specifications in general, as they are currently set, may be unnecessarily severe.